The present invention relates to a method of separating an aromatic or aromatic compound from an entry hydrocarbon mixture having an aromatic content, i.e. containing a mixture of aromatic compounds, and containing nonaromatic components which can include paraffin, cycloparaffin, olefins, diolefins and organic sulfur compounds.
A method of separating an aromatic from an entry hydrocarbon mixture of this type is known. In this known method the separation occurs by an extractive distillation, in which N-substituted morpholine, whose substituents do not have more than seven carbon atoms, is used as selective solvent. The nonaromatic components of the entry hydrocarbon mixture are distilled off the head or top of an extractive distillation column, while the aromatic components are drawn off together with the selective solvent from the sump of the extractive distillation column. The top product is distilled in a separate top product distillation column for recovery of the solvent residue present in it.
The above-described method of obtaining aromatic compounds has been known for many years and has proven satisfactory in the intervening time in a number of different large scale plants, particularly in the case in which N-formyl morpholine is used as a selective solvent. Normally the sump product drawn from the extractive distillation column is conducted into a connected separating column, in which the aromatics contained in it are separated distillatively from the solvent. The solvent drawn from the sump of the separator column is fed back into the extractive distillation column for reuse. Up to now the solvent has been fed to and returned to the extractive distillation column at its head for various technical reasons, so that the top product still contains a certain solvent residue, which can amount to up to 2% by weight. Because of efficiency considerations and to obtain a top product, which is as pure as possible, it is essential that as much of this solvent in the top product be recovered as possible.
Up to now it was standard practice that top product from the extractive distillation column be conducted into a separate distillation column, in which the hydrocarbons of the top product were separated from the solvent. Since the hydrocarbons of the top product must have a solvent content less than 1 ppm, this distillative separation requires a highly expensive apparatus and a high energy consumption.
To decrease the high energy requirement, in German Published Patent Application 34 09 030 it has already been suggested that the distillative separation of the top product from the extractive distillation column be conducted under conditions such that the sump product obtained has a solvent content of from 20 to 75% by weight. Subsequently this sump product is separated in a separating vessel into a light phase and a heavy phase. The solvent rich heavy phase is returned to the extractive distillation column and the solvent poor light phase is fed to a top product distillation column. With this method of course the energy requirements for the purification of the top product are reduced. However, this still requires a separate column for the distillation of top product and also a separating vessel for the separation of heavier and lighter phase, which also means a not inconsiderable apparatus expense.